Frost can be undesirable in a number of applications and in particular, in the airline industry where, globally, approximately ⅓ of all aircraft deicing operations are performed to remove frost from critical aircraft surfaces. At some airports, frost removal can account for over 75% of deicing events.
Frost has a significant negative impact on aerodynamics. The height and close spacing of frost crystals over a wing surface can be such that it disturbs the airflow much more severely than other forms of frozen contamination. The aerodynamic effect of frost on lift is most pronounced when frost is located on or near a wing's leading edge, and the lift loss effect is the most severe for aircraft without leading edge devices. Frost's impact on drag is most severe when it is distributed extensively over an aircraft. In this case, the aircraft is unlikely to achieve a predicted climb gradient. Thus, frost affects an aircraft's performance and handling qualities and can prove to be lethal irrespective of the size of the aircraft involved.
Lift can decrease by as much as 30% with an attendant rise in drag by as much as 40%. Airflow disturbance caused by frost can also result in control difficulties, especially if the frost is asymmetrically distributed from wing to wing, such as might be the case if one wing was in the early morning sunlight and the other was in the shade.
Active frost conditions are defined as the environmental conditions required for the formation of frost. Once active frost conditions have ceased (i.e. environmental conditions do not support frost formation), any remaining frost is defined as passive frost. The FAA defines active frost as frost that actively grows in mass and thickness.
Frost occurs when the surface temperature of an object is below the frost point. Frost point is determined by outside air temperature (OAT) and relative humidity (RH). The ideal conditions for frost accretion are the following:
Outside Ambient Temperature: Below 3° C.
Relative Humidity: Above 60%
Wind Speed: Less than 5 km/h
Sky Condition: Clear, or mostly unobstructed
Active frost occurs on aircraft surfaces that are at or below 0° C. and at or below dew point with dew point being the temperature at which water vapor will begin to condense. Relative humidity at the dew point is 100%.
Determining the onset and completion of active frost conditions is critical to aircraft operations, as active frost conditions require different strategies for aircraft de/anti-icing and pre-flight servicing of the aircraft. Passive frost can be removed from critical aircraft surfaces without concern for reoccurrence and as such anti-icing fluids may not be required. Depending on outside ambient temperatures, it is possible to remove frost without using freeze point depressant fluids, so called deicing or anti-icing fluids, such as propylene glycol, ethylene glycol or others known in the art. Reducing the use of deicing or anti-icing fluid during passive frost removal may lower costs, eliminate or reduce negative environmental impacts, and may result in quicker turnaround time and/or less aircraft delays
In the airline industry, if a flight is departing during active frost conditions, not only does any existing frost have to be removed from critical surfaces, but the pilot must ensure that the fluids applied provide sufficient holdover time (HOT) for the aircraft to depart. In current practice, this requires the use of anti-icing fluids which are expensive, have significant negative environmental impacts and usually result in increased aircraft turn around times.
Within the airline industry, aircraft turnaround time, or the task of getting an aircraft to “wheels up” (safely) is of primary importance as any delays on a single flight could delay the aircraft's subsequent flights for the day and a delay in any flight could result in passengers or cargo missing connecting flights. For this reason, airlines often incur extra costs to reduce the risks of delays in “wheels up” time. With respect to frost conditions, there are no reliable indications for active frost. In the absence of knowledge of whether frost is active or passive, airlines need to make the conservative assumption that the frost is active and to treat it accordingly. At some airports, overnighting aircraft are uniformly coated with anti-icing fluid every night to prevent frost formation.
The Society of Automotive Engineers (SAE) sub-committee AC-9C defines frost as “ice crystals formed on a surface by water vapour deposition from the atmosphere.” Transport Canada expands on this, describing frost as “a thin white deposit of frozen precipitation which is of fine crystalline texture, that adheres to exposed surfaces usually during below freezing, calm winds, cloudless nights with air of high relative humidity and with no precipitation falling. Often the frost deposit is thin enough for surface features underneath, such as paint lines, to be distinguishable.”
Frost forms by deposition of water vapour onto a surface directly from vapour to solid without passing through the liquid stage. Deposition occurs when a surface's temperature falls below the frost point of the air above it. The surface can be cooled to below the frost point in several ways including cold soaking the wing or radiation cooling. The formation of frost does not require 100% relative humidity. In fact, Transport Canada tests suggest that the most rapid frost formation occurs near 75% relative humidity. Testing sponsored by Transport Canada has further revealed that the upper surface of a wing can be as much as 8° C. colder than the ambient air temperature due to radiation cooling at night. Operational crews need to be aware that these conditions are conducive to the formation of frost. The lower the outside air temperature, the more tenaciously the frost appears to adhere to the surfaces.
The frost point (also referred to as ‘icing temperature’ in some material) is similar to the dew point, in that it is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to be deposited on a surface as ice without going through the liquid phase. The frost point for a given parcel of air is always higher than the dew point, at temperatures below 0° C., as the stronger bonding between water molecules on the surface of ice requires higher temperature to break.
Currently no standardized technology or manual approach is being used in aircraft operations to determine active frost conditions. Typically forecast weather conditions are monitored for an indication of whether or not frost formation is likely and the resulting probability is used in planning processes. However, once frost is discovered, usually through visual inspections of aircraft surfaces, there are no methods to reliably determine whether it is active frost (i.e. conditions for continued frost formation persist).
The invention will be used to tell flight crews, airport operators, regulators, other meteorological agencies and other stakeholders about whether or not active frost conditions are present at airport sites.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.